This invention relates to the reversible coding of sequences of 2 or more binary or n-state symbols. More specifically it provides novel methods and apparatus for coding a first plurality of symbols with a second plurality of symbols using at least one reversible logic function and one non-reversible logic function.
In general coding of plurality of symbols comprises using binary symbols, and combining symbols by using a binary reversible logic function, usually the XOR function. The other reversible binary logic function is the EQUAL function.
The use of the XOR (also known as modulo-2 addition) is well known. This means that unauthorized users trying to read a coded digital message of binary symbols can make a reliable assumption that coding of the message uses the XOR function. This facilitates unauthorized decoding of a message. Messages using n-state symbols often are recoded into binary symbols. These messages are often coded by using again the XOR function, for instance by using additions over GF(n=2p), which can be expressed as a plurality of XOR functions.
Accordingly novel and improved methods and apparatus applying a wider range of less predictable functions than only the XOR function for coding messages of binary and n-state symbols are required.